1. Field of the Invention
The present invention relates to solid-state imaging devices.
2. Description of the Related Art
FIG. 1 is a plan view illustrating an example of a known solid-state imaging device. In a solid-state imaging device 102 shown in FIG. 1, a plurality of photoelectric conversion elements 106 are disposed in a matrix with a gap therebetween on a photodetector region 105 of a silicon semiconductor substrate 104. A vertical charge transfer register 108 having a charge coupled device (CCD) structure is disposed for each column of the photoelectric conversion elements 106 in the vertical direction (indicated by the arrow V in FIG. 1 of the photoelectric conversion elements 106. A horizontal charge transfer register 110 also having a CCD structure is disposed at one side of the individual vertical charge transfer registers 108 in the horizontal direction (indicated by the arrow H in FIG. 1) of the photoelectric conversion elements 106. An output portion 112 is formed at one end of the horizontal charge transfer register 110.
In the above-configured solid-state imaging device 102, upon receiving light, signal charges are generated by each column of the photoelectric conversion elements 106, and are supplied to the corresponding vertical charge transfer register 108 via a read area (not shown) which intervenes between the photoelectric conversion elements 106 and each of the vertical charge transfer registers 108. Then, the vertical charge transfer register 108 sequentially transfers the signal charges to the horizontal charge transfer register 110. Upon receiving the signal charges from the individual vertical charge transfer registers 108, the horizontal charge transfer register 110 then transfers the signal charges to the output portion 112. The output portion 112 converts the signal charges into a voltage signal, amplifies it, and outputs the amplified signal.
In the solid-state imaging device 102, in order to obtain a higher level of resolution of captured images, the number of pixels should be increased. To achieve this, it is necessary to dispose more photoelectric conversion elements 106 on the semiconductor substrate 104. However, an increased number of photoelectric conversion elements 106 prolongs the time required for transferring the signal charge, and it becomes difficult to ensure the sufficient frame frequency required for displaying captured images.
In order to overcome the above-described drawback, a solid-state imaging device has been proposed in which the photoelectric conversion elements 106 are divided into two groups, which then transfer signal charges by using two horizontal charge transfer registers. FIG. 2 is a schematic diagram illustrating such a solid-state imaging device. In a solid-state imaging device 114 shown in FIG. 2, a photodetector region 105 on a semiconductor substrate is divided into first and second photodetector areas 116 and 118, and a plurality of photoelectric conversion elements disposed in a matrix are divided into a group disposed in the first photodetector area 116 and a group disposed on the second photodetector area 118. The signal charges generated by the photoelectric conversion elements in the first and second photodetector areas 116 and 118 are respectively transferred to first and second horizontal charge transfer registers 120 and 122 by using the corresponding vertical charge transfer registers. The photoelectric conversion elements and the vertical charge transfer registers are not shown in FIG. 2.
Upon receiving the signal charges from the vertical charge transfer registers, the first and second horizontal charge transfer registers 120 and 122 simultaneously transfer the signal charges in the opposite directions so as to supply them to first and second output portions 124 and 126, respectively, formed at the corresponding ends of the first and second horizontal charge transfer registers 120 and 122. Then, signals indicating images captured by the photoelectric conversion elements disposed in the first and second photodetector areas 116 and 118 are simultaneously output from the first and second output portions 124 and 126, respectively. Accordingly, in the solid-state imaging device 114, the time required for transferring the signal charge is decreased to one half the time for a solid-state imaging device using only one horizontal charge transfer register.
FIG. 3 is a plan view illustrating details of the first and second output portions 124 and 126 disposed at the corresponding ends of the first and second horizontal charge transfer registers 120 and 122, respectively, shown in FIG. 2.
The first and second output portions 124 and 126 respectively include first and second signal charge detectors 128 and 130, first and second transistors 132 and 134, and first and second charge sweeping regions 136 and 138.
Each of the first and second horizontal charge transfer registers 120 and 122 includes a transfer passage 140, which is, for example, an n-type region formed on the surface of a p-type semiconductor substrate, and transfer electrodes (not shown) disposed on the transfer passage 140 in the charge transfer direction. The transfer passage 140 is formed to be narrower, as shown in FIG. 3, as it goes to the end of each of the first and second horizontal charge transfer registers 120 and 122. Each of the first and second signal charge detectors 128 and 130 is formed as, for example, an n-type region on the surface of the semiconductor substrate, at the vicinity of the front end of the transfer passage 140.
Each of the first and second transistors 132 and 134 includes a gate 142, a source 144, and a drain 146. The gate 142 is formed of, for example, polysilicon, and is formed such that one end thereof overlaps with the corresponding signal charge detector 128 or 130. In this example shown in FIG. 3, the gate 142 of the first transistor 132 extends upward toward the upper left side, while the gate 142 of the second transistor 134 extends upward toward the upper right side. The source 144 and the drain 146 of each of the first and second transistors 132 and 134 are formed as, for example, n-type regions on the surface of the semiconductor substrate, across the gate 142.
Each of the first and second charge sweeping regions 136 and 138 includes, as shown in FIG. 3, a charge sweeping drain 150 and a charge sweeping control gate 152. The charge sweeping control gate 152 is disposed adjacent to the gate 142 of each of the first and second transistors 132 and 134, and the charge sweeping drain 150 is formed as, for example, an n-type region on the surface of the semiconductor, at a position opposite to the gate 142 across the charge sweeping control gate 152.
As shown in FIG. 3, the first signal charge detector 128, the first transistor 132, and the first charge sweeping region 136 are symmetrical to the second signal charge detector 130, the second transistor 134, and the second charge sweeping region 138, respectively, with respect to an imaginary line 154 drawn orthogonally to the direction in which the first and second horizontal charge transfer registers 120 and 122 are extended.
With this configuration, signal charges transferred from the first and second horizontal charge transfer registers 120 and 122 are converted into voltage signals having a magnitude according to the amount of charge by the first and second signal charge detectors 128 and 130, and the first and second transistors 132 and 134 amplify the voltage signals and output them from the drains 146.
The charge sweeping control gates 152 are controlled to be ON or OFF in synchronization with the charge transfer operation of the first and second horizontal charge transfer registers 120 and 122, and signal charges which have become unnecessary in the first and second signal charge detectors 128 and 130 are transferred to the charge sweeping drains 150 via the charge sweeping control gates 152.
The first and second transistors 132 and 134 and the first and second sweeping regions 136 and 138 which form the first and second output portions 124 and 126, respectively, are formed on a semiconductor substrate according to a known semiconductor processing technique, which has a plurality of steps. During the processing, a plurality of masks, such as photoresist or silicon nitride film, are formed in each step according to, for example, a photolithographic technique. As a result, the sources 144, the drains 146, and the gates 142 are formed in predetermined configurations at predetermined positions on the semiconductor substrate.
When a mask is formed by using, for example, a photoresist, according to a photolithographic technique, a photoresist film on a semiconductor substrate is exposed via a reticle, and is then developed so as to be formed into a mask pattern. Generally, however, the mask formed as described above is slightly displaced vertically, horizontally, to the left side, or to the right side from a correct position, on the mask surface within a predetermined allowance.
Accordingly, if, for example, a mask for forming the gate 142 of the first or second transistor 132 or 134 is slightly displaced with respect to a mask for forming the source 144 and a mask for forming the drain 146 of the corresponding transistor 132 or 134, the gate 142 is displaced from the correct position with respect to the source 144 and the drain 146. Such a displacement of the gate 142 adversely influences the characteristics of the corresponding transistor 132 or 134, which changes, for example, the degree of amplification of the transistor.
FIGS. 4A and 4B are partial plan views illustrating the gates 142 of the first and second transistors 132 and 134, respectively, which are formed slightly rightward from the correct positions due to displacements of the corresponding masks.
Since the mask for forming the gate 142 is displaced rightward, as shown in FIG. 4A, the gate 142 of the first transistor 132 is also formed rightward. As a consequence, the area of the drain 146 of the first transistor 132 is smaller than it should be, while the area of the source 144 is larger than it should be.
In contrast, as shown in FIG. 4B, since the gate 142 of the second transistor 134 is formed rightward, the area of the drain 146 is larger than it should be, while the source 144 is smaller than it should be.
Accordingly, such displacements of the masks from the correct positions change the characteristics of the first and second transistors 132 and 134 in different manners. For example, if the degree of amplification of one transistor is increased, the degree of amplification of the other transistor is decreased.
If the degree of amplification is different between the first and second transistors 132 and 134, the levels of the signals output from the first and second transistors 132 and 134 also become different. As a result, when captured images are displayed based on these signals, the brightness or the contrast becomes different between the left side and the right side of the screen.
The same applies to the first and second signal charge detectors 128 and 130, and the first and second charge sweeping regions 136 and 138. Thus, the levels of the signals output from the first and second output portions 124 and 126 become different.
Accordingly, in order to solve the above-described problem, it is an object of the present invention to provide a solid-state imaging device in which output portions provided for individual charge transfer registers can be formed without differentiating the electrical characteristics when transferring signal charges generated by photoelectric conversion elements in different directions by the charge transfer registers.
In order to achieve the above object, according to the present invention, there is provided a solid-state imaging device including: a plurality of photoelectric conversion elements; first and second charge transfer registers for transferring signal charges generated by the plurality of photoelectric conversion elements; first and second signal charge detectors, each being disposed in the vicinity of one edge of each of the first and second charge transfer registers, respectively; and first and second transistors. The above-described components are disposed on a semiconductor substrate. The first and second charge transfer registers transfer the signal charges in opposite directions. The first and second signal charge detectors receive the signal charges from the first and second charge transfer registers, respectively, and output voltages having a magnitude in accordance with the amount of charge. Each of the first and second transistors includes a gate into which the voltage output from the first or second signal charge detector is input, a source, and a drain, both of which are formed adjacent to the gate. The first signal charge detector, the source, the drain, and the gate of the first transistor are almost congruent to the second signal charge detector, the source, the drain, and the gate of the second transistor, respectively, when viewed from above. The first signal charge detector, the source, the drain, and the gate of the first transistor are overlaid almost completely on the second signal charge detector, the source, the drain, and the gate of the second transistor, respectively, by a translational movement.